The present invention relates to magnetic resonance systems, and more particularly to RF power amplifiers for parallel excitation in magnetic resonance systems.
There has been very active development in the field of magnetic resonance (MR) where parallel RF transmission with multiple transmit elements is used to benefit various applications by improving spin excitation. In high field MR, inhomogeneity in the RF magnetic field caused by wave propagation and dielectric effect in particular, may be reduced by optimizing the amplitude and phase of the driving currents when conducting multi-port excitation on birdcage coils or transmit arrays consisting of individual coil elements. It is possible to further reduce the RF magnetic field in homogeneity effect by independently controlling the RF waveforms of individual transmit channels and leveraging the capacity of full fledged parallel RF transmission in accelerating multidimensional excitation and managing power deposition.
Recent developments have provided support for direct validation of full-fledged parallel RF transmission principles. However, the development of efficient parallel transmit coil arrays remains a significant challenge. Coupling between the transmit coil elements is one of the key challenges to transmit coil array construction and use. Many approaches have been proposed to address the issue of inter coil coupling. One of the approaches defines, for parallel RF reception, a preamplifier decoupling scheme. This scheme reduces the input impedance of a preamplifier to nearly zero, thereby maximizing the input impedance seen by the corresponding receive coil at its output port and causing blockage of coupled current in the coil. However, for parallel transmit, practicing an analogous scheme is ineffective due to the typical RF amplifiers' 50Ω impedance seen by the coils.
Many decoupling methods have been proposed to address the inter-coil coupling problem. One category of methods introduces partial geometric overlap of coils to annul the mutual inductance between them. Such methods are effective for nearest neighbor elements only, and tend to impose stringent constraints on the geometry and placement of the individual coils. Another category of methods employs a capacitive or inductive decoupling bridge or a multi-port network, at the cost of increased RF loss and increased complexity of the decoupling circuits and tuning efforts. A third category of methods suppresses the coupling-induced currents with high source impedance, by, for example, integrating RF power MOSFET's with the rungs of a TEM coil or driving nonresonant loop-shaped coils directly. In these examples, a MOSFET is configured to function approximately as a current source, and thus to yield high impedance at the driving ports. However, the series resonant element in this method also acts as a severely mismatched load to the MOSFET, which may significantly degrades its maximum available output power. A fourth category of methods applies active decoupling. Such methods calibrate coupling between element coils first and then introduce proper correlations, realized either by analog circuits or a digital vector modulation array, between the driving voltages of each element to cancel the coupling components in the currents.
Therefore, there is a need for a system and method for development of a decoupling method that supports parallel transmit applications and facilitates transmit performance optimization by eliminating constraints on array geometry.